Insulin-like growth factor-l (IGF-I) is one of several skeletal growth factors that circulates and also is highly expressed in bone. Serum IGF-I is a complex trait associated with bone mineral density (BMD) and the risk of osteoporosis. Our laboratory has been using the mouse as a model to help identify heritable determinants of IGF-I. Initially, we found two strains of mice, C57BL/6J (B6) and C3H/HeJ (C3H) with differences in serum IGF-I (i.e. high IGF-I in C3H vs low IGF-I in B6 mice). We reported that transcripts of the P2 promoter from the Igf1 gene in liver of C3H were two fold greater than B6, that IGF-I expression was twice as high in C3H adipose and skeletal tissue compared to B6, and that C3H Igfl mRNA half life was prolonged. We mapped a quantitative trait locus (QTL) for circulating IGF-I on chromosome (Chr) 10. To test the effect of this QTL, we made a congenic mouse (B6.C3H-10T:10T) strain by introgressing the C3H QTL on Chr 10 into a B6 background with 10 backcross generations. 10T mice recapitulated the serum IGF-I phenotype of C3H and their skeletal micro architecture very closely resembled a Col1A1 Igf1 transgenic with increased trabecular number and osteoclasts than B6. We then generated 8 nested congenic sublines, one of which (10-7) showed a 40% increase in serum IGF-I, accompanied by significant changes in trabecular micro-architecture. We were then able to narrow our QTL to 13cM, eliminated several candidate genes including Socs-2, Kitl, and Elk3 and identified the Igfl gene as the most promising candidate. By sequencing several regions of the Igfl gene in B6 and C3H, we found several potentially important polymorphisms, including three within the 3'UTR, one in a potentially important miRNA binding site highly conserved across species. Taken together these lines of evidence strongly support our hypothesis that the Chr 10 QTL is defined by cis polymorphisms in the Igfl gene, and that these alleles determine strain variation in circulating IGF-I and trabecular remodeling. To test that central hypothesis and define the cellular and molecular mechanisms of enhanced IGF-I expression, we propose 3 specific aims, including: 1- Sequencing the entire Igfl gene in 10-7 and B6, and testing polymorphisms from the 5'and 3'UTRs;2- Generating a BAG congenic strain by homologous recombination of a C3H BAG into B6 ES cells and performing comprehensive phenotyping;and 3- Defining the mechanism of increased bone turnover in the 10-7 line and its relationship to IGF-I expression in vitro and in vivo. The public health implications of these studies are significant. IGF-1 is a critical modulator of skeletal acquisition and has been considered a marker for determining fracture risk. By defining the genetic determinants of this peptide, using unique mouse models, we will gain insight into the pathogenesis of osteoporosis and delineate the molecular mechanisms of IGF-I regulation. These studies will also determine how specific alleles in the Igfl gene affect critical post-transcriptional events that ultimately determine circulating levels of IGF-I.